Thuja orientalis, or bookleaf pine, tends to become bare in the center but maintains an attractive green at the tips of the branches. It's normal for this plant for the center foliage to turn brown and then drop away. This is a slow process, however. If yours are quickly dropping foliage, you may want to check for spidermites. These insect pests are tiny and difficult to see but leave webbing where the stems meet the branches. If you find no signs of insect pests, your bookleaf pines are just fine.